Method of forming thin-walled hollow ferrous castings



Feb.z1, 1939. R. H, TERRY .ETV-AL 2,147,724

METHOD oF FORMING THIN-WALLED HOLLOW FERRoUs CASTINGS Patente-d Feb. 21{1'939 lmTHOD F FORMING THIN-WALLED HOLLOW FERROUS CASTINGS Robert H. Terry and Alfred L. Boegehold, Detroit,- Mich., assignors to General Motors Corporation, o Detroitglvlich., a. corporation-of Delaware Application February 7, 1936, Serial No. 62,724

12 Claims. (Cl. Ztl-448.2)

This invention relates to a process for producing thin-walled hollow shapes. Very often it is desired to construct light weight articles ofa complicated shape which `are very difficult to .1 make. Such articles maybe made by'uniting various sheet metal stampings .but this process is undesirable due to `the excessive cost of dies for the parts. We have found a process by which these thin walled objects may be made by a casting and heat treating process.

ln general, the process consists of making a white iron casting similar in composition to the white iron used for makingl malleable cast iron. This casting is then heated to some temperature 15 between 1600 and 2000 F. in'an atmosphere of moist hydrogen for a long enough period to decarburize the surface layer-of the casting to a depth equal to the wall thickness desired in the finished shape. yThis decarburized casting is gm then drilled in one or two places through the decarburized skin. The holes are then filled with a paste of carbonaceous material and the casting is then again heated to a temperature above the melting point of the undecarburized center of o5 the casting. This melts the inside undecarburized material which bleeds from the hole drilled through the decarburized skin, leaving the low carbon outside skin which forms the hollow object. Hollow thin-walled objects made by this o@ process may beused to. advantage as component parts for a copper brazed structure.

rThe wall material resulting from the process is suitable .tor copper brazing because it is very `low in carbon and behaves in the same way as m low carbon steel. It is also free from :Hake

graphite `or free carbon in any formand, thereiore, does not change in size during the brazing operation in the way that cast iron'or malleable iron does. di@ Irthe specific conditions which have been used in making the hollow articles thus far have been as follows:

.a casting has been made containing 21/2% car- V bon, 1.15% silicon and 40% vanadium, .40% am manganese, the rest iron, plus usual impurities suchas phosphorus and sulphur. In order toobtain a decarburized surface layer .040 deep, the casting was heated for one and one-half hours at l800 F. in moist hydrogen. Thinner or thicker layers of decarburized irons may be obtained by varying the time and temperature. After having produced the decarburized surface layer, the casting is cooled down and a hole drilled which is plugged with graphite paste in g55 order to prevent decarburization of -this hole,

when again heated in hydrogen to melt out the in`- side. The casting is then heated between 2100 and 2300 F. and held long enough for the core to melt and drain out. v

Experiments have been conducted in which 5 the original White iron casting` with a drain hole provided in the casting is heated to 1800" F. in the moist hydrogen atmosphere for a period long enough to produce a decarburized layer and then `heated on up to above 2100 to melt out` the core'. l0

decarburization was not uniform, producing less desirable result than Withthe composition, given above. i'

Another test with the above given first composition without the vanadium did not drain sat- 25 isfactorily so that it appears that the presence of the vanadium is instrumental in producing the best results. y

Very good results have been obtained from the use of iron carbon alloy castings.

On the drawings Figure l shows asection through a conventional furnace with the casting of. the invention therein.

Figure 2 is a section through the casting after 35 the outer skin or layer has been decarburized.

Figure 3 is a similar section but showing the .opening or cavity made in the carburized castng. o

Figure 4 is a view similar to Figures 2 and 3 but 40:

showing the graphite plug in the cavity.

Figure 51s a view similar to Figure l but showing the decarburized casting replaced in the furnace and showing the openings with the metal A tube l0 is provided at any suitable point to pro- 55l vide for the inlet of hydrogen gas. The numeral I2 indicates the casting of the invention.

The casting I2 is formed to the shape indicated in Figure 1 by the usual casting methods and prior to its being placed in the furnace is composed of solid cast iron. The particular article in question is the inlet and outlet port for a twocycle engine having a U-type cylinder. The article is heated in the furnace until the outer layer or skin is decarburized.

In Figures 2, 3, and 4, the casting I2 is shown in section and has the openings I4 to enable the inlet and outlet of gas to and from the cylinders. In these figures, the numeral I6 designates the outer decarburized surface or skin of the casting, while the numeral I8 shows the inner metallic or undecarburized portion, the dotted line showing the line of division between the two. As shown in Figure 3, one or more holes or cavities 20 is provided in the casting after the decarburized surface I6 has been formed thereon and this cavity 20 is later lled with the plug indicated at 22. The material of this plug 22 must comprise a substance or material that will prevent the decarburization of the metal, particularly the metal in the hole. Preferably the plug is composed of a non-fusible carbonaceous material such as a graphite paste.

After the graphite plug or plugs 22 have been applied, the casting is replaced in the furnace as shown in Figure and reheated in an atmosphere of hydrogen or other non-oxidizing atmosphere. The graphite plug will prevent further decarburization of the interior metal at the hole. The temperature is increased until it passes somewhat beyond the fusion point of the metal I8 on the interior, but is never .allowed to get high enough to melt the exterior decarburized shell I6. When the desired temperature' has been reached the carbonaceous plug 22.will be forced out by the pressure of the molten metal I8 and allow the metal I8 of the interior to ilow from the openings, the melted metal being indicated by the numeral 24 in Figure 5. When the interior metal is fused it will have a small amount of expansive force due to its change from the solid to the liquid state. This force probably aids in the pushing out of the graphite plug.

After the metal 24 has fused and flowed from the opening or cavity 20 the article isv removed and allowed to cool and is then in the shape shown in Figure 7. It willbe noted that the casting is hollow, leaving a thin-shelled casting through which water may be freely circulated if desired. The interior or hollow portion 26 in Figures 6 and 7 represents the portion previously occupied by the metal I8 in Figures 2, 3, and 4.

When the hollow castings are taken from the furnace it has been found that at times some of them are very brittle and break easily. This is due to the carburizing action of the undecarburized molten metal from the interior of the casting. When the article is heated to cause the interior metal to flow there is sometimes a carburizing action causing the carbon to penetrate the decarburized surface. This causes the brittleness. The article or casting can subsequently be made vductile by again placing it in a furnace and decarburizing in an atmosphere of moist hydrogen.

The hollow article of the invention may be made `without removing the article from the furnace in order to make the hole or cavity.' 'Ihis g is accomplished by making a cavity in the casting before it is put into the furnace. This cavity is then filled with some kind of a material, such as graphite, which is not affected by the heat necessary to decarburize the outer surface of the casting. By controlling the temperature and causing the temperature gradually to rise so that the hydrogen atmosphere will decarburize the exterior, and by then bringing the temperature to a suiliciently high degree,"t-he interior metal will fuse and the weight of the fused metal will push out the plug and allow the metal to flow from the casting. After the metal has run out the process may be continued for a short time thoroughly to decarburize the hollow article.

While Athe invention has been shown applied to one specic article, it is capable of use in connection with other articles Where a light casting is desired.

We claim:

1. The method of forming a thin-shelled karticle or casting consisting of heating an.iron carbon alloy casting in an atmosphere of moist hydrogen to cause the decarburization of the surface of the casting, allowing the casting to cool, making a cavity or hole in the casting, lling the hole with a substance capable of preventing the decarburization of the casting at the hole, then in reheating the casting to a temperature above the fusion temperature of the metal inside the casting to cause the molten metal to force the lled substance from the casting to enable the metal to flow and leave a hollow article.

2. The method of forming a thin-shelled casting consisting of heating a casting in an atmosphere of moist hydrogen to a temperature to cause the decarburization of the surface of the casting, allowing the casting to cool suiliciently to enable the placing of a hole or cavity therein, making a cavity or hole in the member, filling the cavity with a material capable of preventing the decarburization of the casting at the hole, then in reheating the casting in a hydrogen atmostphere to a temperature which is lower than the fusion point of the decarburized surface but higher than the fusion point of the interior undecarburized metal thereby to cause the undecarburized metal to fuse and force the lled material from the hole to enable the metal to flow from the casting and leave a hollow article.

VV`3. The method of forming a thin-shelled article consisting of decarburizing the surface of a member, allowing the member to cool, making a cavity or hole in the member, lling the hole witha substance capable of preventing the decarburizationof the member at the hole, then in heating the member in a non-oxidizing atmosphere to a temperature lower than the fusion temperature of the decarburized metal but greater than the temperature of fusion of the metal inside the member to cause the weight of the molten interior metal to force the illed substance from the hole to enable the interior metal to flow from' the member and leave a hollow article.

4. The method of forming a thin-shelled article consisting of heating a metallic member in an atmosphere of moist hydrogen to a temperature to cause the decorburization of the surface of the member, allowing the member to cool sufciently Ato enable the placing of the hole or cavity therein, making a -cavity or hole in the member, lling the cavity with a material capable of preventing the decarburization of the member at the hole, then in reheating the member in a hydrogen atmosphere to a. temperature which is lower than the fusion point of the deventing the decarburization of the casting at' the hole, then in reheating the casting in a hydrogen atmosphere to a temperature which is carburized surface but higher than the fusion` point of the interior metal to cause the molten metal to force the filled material from the hole to enable the metal to flow from the casting and leave a hollow article.

5. The method of forminga thin-shelled casting consisting of heating a casting in an atmosphere of moist hydrogen to a temperature to` cause the decarburization of the surface of the casting, allowing` the casting to cool sumoiently to enable the making of a hole or cavity therein, making a cavity or hole in the member, filling the cavity with a material capable of prelower than the fusion point of the `decarlcurized surface but higher than the fusion noint of the interior undecarburized metal thereby to cause the undecarburzed metal to fuse and force the iilled material from the hole to enable the metal to How from the casting and leave a hollow article and then reheating the hollow article in a non-oxidizing atmosphere to decarburize the article.

6. The method of forming a thin-shelled article consisting of decarburizing the surface of a member, allowing the member to cool, making a cavity or hole .in the member, lling the hole with a substance capable of preventing the decarburization of the member at the hole, then in heating the member in a non-oxidizing atmosphere to a temperature lower than the fusion temperature of the decarburized metal but greater Athan the temperature of fusion of the metal inside the member to cause the weight of the molten interior metal to force the filled substance from the cavity to enable the interior metal to ow from the member and leave a hollow article, and then reheating the hollow article in a non-oxidizing atmosphere to decarburize the article.

7.. The method of forming a thin-shelled article consisting of heating a metallic member in an atmosphere of moist hydrogen to a tempera-` ture to cause the decarburization of the surface of the member, allowing the member to cool suiiiciently to enable the making of the hole or cavity therein, making a cavity or hole in the member, iilling the hole with a material capable of preventing the decarburization of the member atthe hole, then in reheating the member in a hydrogen atmosphere to a temperature which is lower than the fusion point of the decarburized surface but higher than the fusion point of the interior metal to cause the molten metal to force the lled material from the cavity to enable the metal to flow from the casting and leave a hollow` article, and then reheating the hollow article in, a non-oxidizing atmosphere to decarburize the article. l

` 8. The method oi forming a thin-shelled arl ticle or casting consisting of forming a cavity or recess in a casting, lling the cavity with a difcultly fusible or non-fusible substance, then in gradually heating the casting in an atmosphere of moist hydrogen to cause the decarburization of the surface of the casting, then in raising the temperature of the casting to a temperature above the fusion temperature of the metal inside the casting to cause the molten metal to force the filled substance from the casting to enable the metal to ow and leave a hollow article.

9. The method of forming a thin-shelled casting consisting of forming a cavity or recess in the casting, filling the cavity with a diiiicultly fusible or non-fusible substance, then in gradually heating the casting inan atmosphere of moist hydrogen to a temperature to cause the decarburization of the surface of the casting, then in heating the casting to a temperature which is lower than the fusion point of the decarburized surface but higher than the fusion point of the interior undecarburized metal thereby to cause the undecarburized metal to :fuse

and force the iilled material from the cavity tov enable the metal to flow from the casting and `leave a hollow article.

. sion temperature of the decarburized metal but greater than the temperature of fusion of the metal inside the member to cause the weight of the molten interior metal to force the lled substance from the cavity to enable the interior metal to flow from the memberl and leave a hol-V low article.

11. The method of forming a thin-shelled ferrous article consisting of forming a cavity or recess in a ferrous member, filling the cavity with a difcultly fusible or non-fusible substance, then in gradually heating the member in an atmosphere of moist hydrogen to a temperature to cause the decarburization of the surface of the member, then in heating the member in the hydrogen atmosphere to a temperature which is lower than the fusion point of the decarburized surface but higher than the fusion point of the interior metal to cause the molten metal to force the filled material from the cavity to enable the metal to flow from the casting and leave a hollow article.

12. The method of forming a thin-shelled article or casting consisting of heating an iron carbon alloy casting in an atmosphere of moist hydrogen to cause the decarburizatlon of the `surface of the casting, allowing the casting to cool, making a cavity or hole in the casting, illing the hole with graphite, then in reheating the casting to .a temperature above the fusion temperature of the metal inside the casting to cause the molten metal to force the graphite from the casting to enable the metal to ow and leave a hol-V low article.v

^ g ROBERT H. TERRY.

ALFRED L. BOEGEI-IOLD. 

